Mysterious Sounds Reported Around the World

People around the world have reported hearing strange sounds from the skies over the past month. Sometimes they describe it as a hum or low rumble; other times it’s a whine, thump, or even a melody. Often the sounds have been recorded and posted online, fueling rumors and conspiracy theories.
One blogger wrote, "either the world is ending, aliens are landing or everyone is getting hoaxed. Or, possibly, there’s an actual scientific explanation for the mass amount of YouTube videos capturing bizarre sounds that are being heard around the globe. Are we witnessing the beginning of a full-scale alien invasion?".
The explanations are almost as varied as the sounds themselves. There's not a single blanket explanation for all the mysterious sounds, though many have been identified. For example there's the recent "midnight roar" reported in Malaysia:

According to a Borneo Post report, the "Sky Roar" had been heard over Kota Samarahan from around 2am or 3am till dawn on both days. Terrified residents, the report added, described the noises as a "loud hushing" or "snoring" sound. The sounds were also recorded that night, and were later uploaded on YouTube. The Minister of Science, Technology and Innovation has a very simple explanation – it was created by an oil palm factory testing their boiler pressure on Jan 11 and Jan 12.

In other cases the strange sound is still being researched; last week the Canadian government was asked to investigate a low-frequency hum that has intermittently plagued citizens in Windsor, Ontario for months.